JP4762878B2 - Weatherproof steel with enhanced rust stabilization ability and method for producing the same - Google Patents

Description

  The present invention relates to a weather-resistant steel material that enables non-painting of steel structures such as bridges and can achieve minimum maintenance, and in particular, promotes the densification of rust on the surface of steel materials for long-term rusting. The present invention relates to a technology for improving the stabilizing ability.

The first weather-resistant steel that was commercialized in the United States in 1933 was introduced in Japan in the 1960s. The SMA weather-resistant steel specified in JIS G3114 (hereinafter abbreviated as JIS-SMA material) It is still widely applied to steel structures where there is a high need for minimum maintenance.
However, some of the examples of application of JIS-SMA materials in Japan that are affected by salt damage at high humidity caused abnormal corrosion that was different from the expected state.

Therefore, as described in Non-Patent Document 1, the use of JIS-SMA material is limited to areas where the amount of incoming salt is 0.05 mg-NaCl / dm ² / day (hereinafter abbreviated as mdd). Accordingly, there has been a strong demand for weathering steel that can be used even in areas where the amount of incoming salt exceeds 0.05 mdd, in order to minimize the maintenance of steel structures.
After decades of theoretical and empirical research (see, for example, Patent Document 1 and Non-Patent Document 2), nickel-based high weathering steel (seaside weathering steel) was commercialized for the first time in 1998. Used for bridges.

In the JIS-SMA material, the action of forming dense protective rust mainly by Cu and Cr is utilized, and the effect of reducing the corrosion rate due to long-term exposure is manifested. On the other hand, when salt damage becomes severe beyond the above-mentioned range, it is reported that Cr accelerates corrosion because it lowers the pH of condensed water at the steel / rust interface (see Non-Patent Document 2).
In order to eliminate this instability, Cr was not added, and referring to the standard of JIS-SMA material, increasing the amount of Ni while preserving Cu, which improves the adhesion of protective rust, increased salt damage resistance. Is nickel-based high weathering steel.

  While the main function of protective rust formed on JIS-SMA material was to improve adhesion and environmental barrier properties, the characteristic of protective rust formed on nickel-based high weather resistant steel is the steel / rust interface. In addition, a pH control function is added. Even if the influence of salt damage is strong, there is no Cr, which is the main cause of corrosion acceleration by lowering the pH, and Na ions are concentrated in the inner layer of rust by the cation exchange function of protective rust enhanced by Ni concentration, and even when wet, steel Since the / rust interface is maintained at a high pH and passivation is ensured, the progress of corrosion is suppressed.

  This nickel-based high weathering steel is defined as a weathering steel in which Ni is added in excess of 0.4%, which is the upper limit content in the JIS-SMA material (see, for example, Non-Patent Document 3). In addition, commercialized nickel-based high weathering steel has different additive element configurations for each manufacturer, Cu-Ni-based high weathering steel, Ca-Cu-Ni-based high weathering steel, N-Cu-Ni-based steel It is classified into high weathering steel, ultra low C-Cu-Ni high weathering steel, Mo-Ni high weathering steel, Ti-Cu-Ni high weathering steel, etc. (For example, refer to Patent Documents 1 to 10).

  In this way, even if it is nickel-based high weather resistant steel, the composition of additive elements is diverse and the history is also short, so the effects and effects of trace additive elements on the corrosion resistance of various nickel-based high weather resistant steels are fully systematized. There is a high possibility that the chemical reaction related to trace elements at the corrosive interface is completely different due to differences in the composition of additive elements, so the effects and effects of trace elements are examined for each additive element composition. There is a need to.

Against this backdrop, each element of C, Si, Mn, P, S, Cu, Ni, (Cr), Mo, and Ti is the Tokyo Institute of Technology Creation Project Research Body SIG1 (high weather resistant steel bridge) With the cooperation of the Japan Iron and Steel Institute, Bridge Research Society, and Weatherproof Steel Corrosion Design WG, the calculation method of the weathering alloy index V shown in the following formula (1) was suggested.
V = 1 / {(1.0-0.16 [C]), (1.05-0.05 [Si]), (1.04-0.016 [Mn]), (1.0-0.5 [P]), (1.0-1.9 [S]) (1.0-0.10 [Cu]), (1.0-0.12 [Ni]), (1.0-0.3 [Mo]), (1.0-1.7 [Ti])} (1)
However, the range of each alloy component of the above formula (1) is mass%, 0 ≦ [C] <1.5, 0.1 <[Si] <5, 0.1 <[Mn] <10, 0 ≦ [P] <0.15 , 0 ≦ [S] <0.03, 0 ≦ [Cu] <1.1, 0 ≦ [Ni] <5, 0 ≦ [Mo] <0.6, 0 ≦ [Ti] <0.12, and V The value range is used after confirming that 0.9 ≦ V ≦ 2.5.

  As a result, as long as these components are used, it can be said that the high salt resistance of the nickel-based high weathering steel can be generally grasped by the V value.

  In addition, the merits and demerits of Cr addition of several percent or less in a salt damage environment have not yet been clarified, and the nickel-based high weathering steel materials that have been commercialized so far eliminate uncertainties that cannot be controlled against salt damage. Therefore, in general, Cr is not added. The reason why Cr does not affect the formula (1), which is the calculation formula of the weather resistant alloy index V, is that it is applied to the Cr-free steel in principle. However, if the amount of impurity Cr inevitably mixed in or the amount of Cr contained in the JIS-SMA material is allowed, the inclusion of the inclusion does not cause a large error.

In this way, even if it is nickel-based high weathering steel, the composition of additive elements is diverse, and it is difficult to grasp the weathering performance only with chemical components, but it has been studied so far to improve the weatherability of steel materials. As has been suggested from the above formula (1), all of them have further added an alloy element other than S.
However, the role of Cu and Ni is relatively clear. However, when other additive elements are added, the corrosion reaction mechanism and the rust formation process are different. The effects may be reversed, and adding elements to the dark clouds only increases costs, and there is a limit to improving the weather resistance.

"Road Bridge Specification and Explanation", Japan Road Association, March 2002 Hiroshi Kihira, Satoshi Ito, Shigeru Mizoguchi, Tomomi Murata, Akira Usami, Yasuko Tanabe: “Materials and Environment” Vol. 49, pp. 30-40 (2000) Chiaki Miki, Atsushi Ichikawa, Makoto Ukai, Masahiro Takemura, Takenori Nakayama, Hiroshi Kihira: "Journal of Civil Engineers" No.738 / I-64, pp.271-281, 2003 Japan Iron and Steel Association, Joint Research Group, Steel Analysis Subcommittee: “Analytical Technology in the Japanese Steel Industry” (1982) Japanese Patent No. 2572447 Japanese Patent No. 3785271 Japanese Patent No. 3817152 Japanese Patent No. 3655765 Japanese Patent No. 3568750 Japanese Patent No. 3458762 Japanese Patent No. 3463600 Japanese Patent No. 3465494 Japanese Patent No. 3646512 Japanese Patent No. 3719053

  Therefore, in the present invention, in the weathering steel, the performance of the weathering steel, in particular, the long-term rust stabilization ability is obtained by using elements and compounds that do not increase the cost in place of the performance improvement by the conventional trace amount additive element. The issue is to improve.

  In the course of investigating the means for improving the performance of weathering steel without adding trace elements, which increases the cost, the present inventors have a 1.2% Ni-0.8% Cu base containing about 0.007% oxygen. In the nickel-based high weathering steel, 0.03% of aluminum was added, but the corrosion amount was found to be minimal. When this phenomenon was investigated in detail, it was found that aluminum nitride having a particle major axis length of 1 μm or less was formed by the addition of a small amount of aluminum, thereby exhibiting a corrosion inhibiting effect.

  Conventionally, nitrogen combined with aluminum has been known to form fine precipitates of aluminum nitride and effectively work to reduce the austenite grain size at high temperatures, which makes it possible to mechanically resist weathering steel. The characteristics can be improved. However, there has been no study on the effect of aluminum nitride on the ability to stabilize rust. Therefore, the present inventors examined the effect of adding aluminum nitride to weathering steel.

It is known that the relationship between the aging corrosion amount Y and the elapsed years X of the weathering steel material is represented by the following formula (2), where A is the initial year corrosion amount and B is the rust stabilization index.
Y = A · X ^B (2)

The rust stabilizing ability as used herein refers to the ability to reduce the corrosion rate over a long period of time by forming a denser rust. Specifically, it can be quantified as the ability to reduce the rust stabilization index (hereinafter referred to as B value) in equation (2).
The inventors of the present invention performed evaluation using the formula (2), and confirmed that the aluminum nitride finely dispersed in the weathering steel has a rust stabilizing ability.

In other words, aluminum nitride was added to the weathering steel and subjected to a long-term exposure test. Using the obtained data, the A value and B value of the formula (2) were evaluated and analyzed by statistical regression. As a result, it was difficult to find a significant difference in the first year corrosion amount (A value), but it became clear that the B value became smaller by the addition of aluminum nitride.
As a result, a new finding that the ability to stabilize rust increases when aluminum nitride is added to weathering steel has been made, and the following present invention has been made.
The gist of the present invention is as follows.

(1) In mass%,
C: 0.03% to 0.18%,
Si: 0.1% to 0.65%,
Mn: 0.2% to 1.4%
P: 0.03% or less S: 0.02% or less,
Cu: 0.3% to 2%,
Ni: 0.2% to 6%,
N: 0.002% to 0.01%,
Al: 0.01 to 0.5%,
Rust stabilization characterized by containing 5 to 50 ppm by mass of aluminum nitride containing 0.005% or less of O 2, the balance Fe and inevitable impurities, and a particle major axis length of 0.001 to 1 μm Weatherproof steel with enhanced performance.
(2) In mass%,
P: more than 0.03% to 0.2%,
Cr: 0.1% to 0.75%
Mo: 0.1% to 0.5%,
Ti: 0.001% to 0.03%
The weatherproof steel with enhanced rust stabilization ability according to (1), characterized by containing at least one of them.
(3) In mass%,
V: 0.001% to 0.05%,
Nb: 0.001% to 0.05%,
W: 0.001% to 0.05%
The weatherproof steel with enhanced rust stabilization ability according to (1) or (2), characterized by containing at least one of the above.
(4) In mass%,
Ca: 0.0001% to 0.005%,
Mg: 0.0001% to 0.005%
The weatherproof steel with enhanced rust stabilization ability according to any one of (1) to (3), characterized by containing one or two of the above.

(5) The method for producing weatherable steel with enhanced rust stabilization ability according to any one of (1) to (4), wherein aluminum nitride is added to molten steel for production.
(6) Aluminum or nitrogen is added to the molten steel after deoxidation, or aluminum, nitrogen and aluminum nitride are added to produce the steel, according to any one of (1) to (4) For producing weatherable steel with improved rust stabilization ability.

  According to the present invention, it is possible to provide a weather-resistant steel material with enhanced rust stabilization ability without increasing the cost. As a result, the long-term corrosion depletion amount of unpainted weather-resistant steel bridges is further reduced, so that a safer and more secure minimum maintenance steel structure can be realized. In addition, since the range of application by using non-painting of weather resistant steel is expanded, the life cycle cost can be reduced in a wider area, and the taxpayer's burden is reduced in the maintenance cost of public steel structures.

Hereinafter, embodiments of the weathering steel of the present invention will be described.
First, the point that aluminum nitride, which is a feature of the present invention, is contained in weathering steel will be described.

  The fine aluminum nitride acts on the nucleation process in the rust formation process and forms a finer rust. The point of the present invention is that both the environment blocking function of the rust layer and the control function of the rust / steel interface pH are enhanced by aluminum nitride. The fine aluminum nitride is ionized by the corrosion of the steel material during the corrosion process, and gradually dissolves in the aqueous solution to provide an appropriately sized site as a fine rust nucleus.

  Nitrogen in aluminum nitride chemically changes to ammonium ion or nitrite ion in an aqueous solution, so that the local pH in the region around the nucleation site is increased, the solubility of iron ions is lowered, and nucleation of rust colloid is accelerated. When nucleation of rust colloid is promoted, fine rust colloid particles of various sizes that effectively fill the voids in the rust layer constituted as aggregates of rust colloid particles are effectively generated. As a result, in the long term, voids in the rust layer are filled with rust colloidal particles of various sizes, contributing to densification of the rust layer. As a result, the rust stabilization ability of the weather resistant steel is enhanced. Since this effect reduces the B value in the above-described equation (2), the long-term corrosion amount suppressing effect becomes enormous.

In order to express this effect, it is necessary that 5 mass ppm or more of aluminum nitride that can function as a nucleation site of rust colloid exists in steel. In order to make the effect more apparent, it is desirable to be present at 10 ppm by mass or more.
In addition, the upper limit particle long axis length of aluminum nitride in steel that can function as a nucleation site of rust colloid (the aluminum nitride particles generally have a needle-like form, and in order to define the size, the particle long axis The length is used) is 1 μm or less, preferably 0.5 μm or less, and more preferably 0.3 μm or less.
The lower limit particle major axis length of aluminum nitride is 0.001 μm or more. When the particle major axis length is less than this, dissolution and disappearance of aluminum nitride is fast, and it hardly functions as a nucleation auxiliary site for rust colloid in the rust formation process. Desirably, it is 0.005 μm or more.

  When such aluminum nitride is dispersed in the steel, the rust stabilizing ability of the weather-resistant steel is enhanced, but excessive addition of aluminum nitride lowers the toughness of the steel material or lowers the hot workability. From this viewpoint, it is desirable that the upper limit of the aluminum nitride content be 50 mass ppm.

  In order to form aluminum nitride as described above in steel, there are a method based on a chemical reaction in molten steel, a method in which aluminum nitride is added to molten steel, or a method in which these methods are combined.

In order to form aluminum nitride by a chemical reaction in the molten steel, Al is added in combination with N addition or the like. At that time, it is necessary to regulate the amount of oxygen in the molten steel low.
The present inventors have found that when the amount of oxygen in the nitrogen-containing steel is restricted to a low level in the aforementioned research process, the weather resistance tends to improve as the amount of aluminum added increases. When the amount of oxygen is large, coarse aluminum oxide having a particle major axis length exceeding 1 μm is formed. Such oxides are believed to accelerate pitting corrosion as a starting point for pitting.

  This effect of improving corrosion resistance is more clearly manifested when the amount of Al added is 0.01% when oxygen is 0.005% or less, and is saturated at 0.48%, but further restricts oxygen to less than 0.001%. In such a case, the weather resistance performance monotonously improved as the Al addition amount increased. However, in order to realize a state of less than 0.001% oxygen in the actual steelmaking process, it is necessary to add a large amount of a strong deoxidizer, leading to an increase in deoxidation cost. For this reason, the upper limit of Al was made 0.5%.

  Thus, when aluminum nitride is formed by a chemical reaction, the key to the present invention is to suppress the formation of aluminum oxide, which has been an impediment to improving weather resistance, and to bring out the effect of aluminum nitride.

  In order to lower the oxygen content while controlling the nitrogen content in the molten steel to a predetermined level in the steelmaking process, such as bubbling high-purity nitrogen gas in the ladle refining process, Consideration is necessary. In a normal steelmaking method using aluminum as a deoxidizer, a large amount of aluminum oxide remains in the steel. In producing a steel material using the weathering steel of the present invention, deoxidation is basically performed before adding aluminum or nitrogen so that aluminum oxide does not remain in the steel material as much as possible. When aluminum is unavoidably used for deoxidation, a sufficient amount of time is taken after deoxidation treatment to float the coarse aluminum oxide formed in the molten steel, and then nitrogen and aluminum are added to form fine aluminum nitride into the steel. It is good to form inside. If the above-mentioned consideration is not made at the manufacturing stage, even if it is a similar component, the effect of the present invention is not exhibited.

In order to ensure the formation of aluminum nitride in the steel, the following method of adding aluminum nitride into the molten steel at the steel making stage is effective.
As a method of addition, a method in which aluminum nitride powder is mixed and injected into an oxygen lance in a converter, a method in which aluminum coated wire is added during ladle refining, and an argon gas to be bubbled during vacuum degassing. There are a method of adding by mixing powder, a method of adding with aluminum-coated wire during continuous casting, a method of placing a predetermined amount of aluminum nitride powder pre-aluminized in the mold before casting, etc. It may be used. Since various methods of adding aluminum nitride to molten steel are conceivable, the method is not limited to the one exemplified here.

Even when aluminum nitride is added to the molten steel, as described above, it is added even when aluminum oxide is formed in the molten steel while the formation of aluminum oxide is suppressed as much as possible. It is preferable to add the coarse aluminum oxide after sufficiently floating.
Further, as a method of forming aluminum nitride in steel, it is needless to say that a method in which the method of adding aluminum nitride and the method of chemical reaction in the molten steel are combined may be used.

Next, the chemical composition of steel as a base containing aluminum nitride will be described. In addition,% of content means the mass%.
C is an essential element for giving a weathering steel a predetermined strength. Moreover, C is an element which improves the weather resistance of steel materials as also in said Formula (1). In view of these, the lower limit was set to 0.03%, and 0.18% was set to the upper limit as a range in which the toughness did not decrease.

  Si is a basic element used for deoxidation during refining. Moreover, Si is an element which improves the weather resistance of steel materials as also in said Formula (1). In view of these, the lower limit was set to 0.1%, and 0.65% was set to the upper limit as a range in which the toughness and weldability did not decrease.

Mn is a basic element that increases strength and improves workability. Further, Mn is an element that improves the weather resistance of the steel as shown in the formula (1). In view of these, the lower limit was set to 0.2%, and 1.4% was set to the upper limit as a range in which the toughness and weldability did not decrease.
P is added to 0.03% or less from the viewpoint of weldability or is not added.

  Since S is an element that lowers the weather resistance as shown in the above formula (1), it is not added. When hot metal desulfurization is insufficient, there is a possibility of contamination as an inevitable impurity, so 0.02 mass is set as the upper limit. Desirably, the production is controlled to 0.01% or less, more preferably 0.005% or less.

  Since Cu is an element that improves the weather resistance, as described above and in the formula (1), 0.3% is set as the lower limit. The upper limit is set to 2% so that hot working cracks do not occur.

  Ni is a basic element that improves the weather resistance in an environment with salt damage as described above and in the formula (1). Since the present invention is effective for general weathering steel having a low Ni content, the lower limit is defined as 0.2%. The definition of nickel-based high weather resistance steel is that Ni is contained in 0.4% or more, but the lower limit is 1% as a desirable range. This is to make a significant difference in weather resistance from the JIS-SMA material or its equivalent steel type. The upper limit is set to 6% in consideration of cost.

  Oxygen O needs to be made 0.005% or less in order to reliably exhibit the effects of the present invention. As described above, the aluminum component in the steel is combined with oxygen to suppress the adverse effect of coarse aluminum oxide that adversely affects the weather resistance, and the effect of improving the rust stabilization ability by the aluminum nitride that is the point of the present invention is maximized. This is because of From the viewpoint of improving rust stabilization ability, the oxygen content in the steel is preferably as low as possible, preferably 0.004% or less, and more preferably 0.003% or less. However, excessive deoxygenation is undesirable because it increases the cost of steelmaking. Since the oxygen content in the steel includes the amount of oxide, the above-mentioned attention is required in the steel making process so that various additive elements used for deoxidation do not remain in the steel as oxides.

N can be added in combination with Al addition or the like in order to form aluminum nitride particles having a major axis length of 0.001 μm or more and 1 μm or less in the steel as described above. Moreover, it contains in steel as a result of adding aluminum nitride in steel.
In order to contain the necessary aluminum nitride, the lower limit of the nitrogen content is defined as 0.002%. However, since the higher the content, the more effective the weather resistance is, so 0.004% or more is desirable, more desirably 0. 0.006% or more is preferable. However, the excessive nitrogen content deteriorates toughness and weldability and increases the cost by adding an excessive load to the manufacturing process. Therefore, the upper limit is defined as 0.01%.

  Since Al forms aluminum nitride by a chemical reaction in the molten steel, it can be added in combination with N addition or the like. Moreover, it contains in steel as a result of adding aluminum nitride in steel. The content range is set to 0.01% as the lower limit in order to contain the required amount of aluminum nitride. Further, the upper limit is set to 0.5% because the effect is saturated as described above.

  The present invention is based on a steel composed of the above components and the balance Fe and unavoidable impurities, and this steel contains aluminum nitride as described above. In addition to the above components, P: 0.0. More than 03% to 0.2%, Cr: 0.1% to 0.75%, Mo: 0.1% to 0.5%, Ti: 0.001% to 0.03%, V: 0.001 % To 0.05%, Nb: 0.001% to 0.05%, and W: 0.001% to 0.05% can be contained in the steel.

  As described above, P is limited in terms of weldability from the viewpoint of weldability, but is also an element that improves the weather resistance as shown in the formula (1), so in applications where weldability is not required so much. It is desirable to add over 0.03%. The reason why the upper limit is regulated to 0.2% is that the limit of weldability is taken into consideration in view of the use of weathering steel.

  As described above, Cr is an element that causes an unstable factor in the corrosion resistance of steel materials, such as deterioration or improvement of weather resistance when used in an area with salt damage. In an environment where salt damage is severe, it is advisable to add no additives in order to eliminate this instability. On the other hand, when used in areas where salt damage is not so severe, such as inland areas, Cr contributes sufficiently to the formation of dense protective rust, and the addition of Cr increases the probability of improving weather resistance. . When used at a point where the influence of salt damage is not so severe, the effect of improving weather resistance is manifested by adding 0.1% or more of Cr. In addition, when adding Cr, the upper limit of Cr amount was allowed to be added up to 0.75% or less which is the upper limit of the standard of JIS-SMA material.

  When Mo is added, the addition of steel is not desirable because it increases the cost of the steel material. However, since Mo is an element that improves the weather resistance as shown in the formula (1), when higher weather resistance is required, It is desirable to add 0.1% or more. Excessive addition becomes a hindrance to the effect of the present invention, so 0.5% is set as the upper limit.

  Ti is known as an element that improves the weather resistance as shown in the formula (1), but excessive addition acts as an inhibiting factor for the effect of the present invention. This is because titanium is easily combined with nitrogen in the molten steel and inhibits formation of aluminum nitride. Since addition may be necessary for improving weldability, the lower limit is set to 0.001%, and the upper limit is set to 0.03% which does not adversely affect the present invention. In order to ensure the effect of the present invention, it is preferably 0.02% or less, more preferably 0.01% or less.

  About V: 0.001% -0.05%, Nb: 0.001% -0.05%, W: 0.001% -0.05%, the effect of this invention is not inhibited especially, Since it is effective in improving the mechanical properties of steel, it can be added when judged useful. However, since excessive addition presents various problems in the manufacturing process and production / processing process, an upper limit value was set for each.

In the present invention, in addition to the above, one or two of Ca: 0.0001% to 0.005% and Mg: 0.0001% to 0.005% can be contained.
Since these additions have a stronger ability to combine with oxygen than aluminum in molten steel, the formation of aluminum oxide is suppressed by suppressing the formation of aluminum oxide, so that the effect of the present invention is not inhibited. In addition, both Ca and Mg generate hydroxide during the corrosion process and drive the increase of the corrosion interface pH. By using together with the dense rust generating effect of the present invention, it is possible to further improve the weather resistance of the steel material. Of the inclusions in the steel formed by the addition of Ca, calcium oxide stably exhibits a pH increasing effect, followed by calcium / aluminum composite oxide. It is desirable that at least one of them is dispersed in the steel.

  Examples of the present invention will be described below, but the examples are examples for confirming the feasibility and effects of the present invention, and the present invention is not limited to these examples.

Tables 1 to 8 show the results of performing exposure tests on steel materials having various chemical components. Table 1 is 3Ni-0.35Cu system, Table 2 is 1.2Ni-0.8Cu system, Table 3 is 0.7Ni-0.5Cu system, Table 4 is 1.2Ni-2.0Cu system, Table 5 is 4 0.0Ni-0.4Cu series, Table 6 is an example of 0.3Ni-0.3Cu series, Table 7 is a 5Ni-0.4Cu series, and Table 8 is an example of 6Ni-0.3Cu series weather resistant steel.
In addition, what is indicated as-in the table indicates that the content was not analyzed because the element was not added.

  In the exposure test, a steel material cut horizontally to 100 mm × 150 mm × t6 mm under a covered frame was attached as a test piece. This test piece is a slab of various components melted using a vacuum induction heating furnace, heat-treated at 1200 ° C. for 5 minutes, rolled into a thickness of 8 mm by a hot rolling mill, and further machined to a predetermined size. It was obtained by processing. At the start of the test, a sand blast treatment was performed in advance to obtain a uniform surface of Sa3 defined in ISO8501-1, and then ultrasonic cleaning was performed with distilled water and acetone to remove surface adhesion.

  The exposure test is conducted for 10 years, and the specimens are collected every 3 years, 5 years, 7 years, and 10 years. For example, the traditional pickling method described in Non-Patent Document 3 is used. The rust was carefully removed so that the weight loss of the blank was minimized, and the weight loss of the steel material was strictly measured to calculate the corrosion loss.

  The exposure location is Futtsu City, Chiba Prefecture, and the annual average amount of incoming salt measured in a covered exposure stand according to JIS Z2381 (General Rules for Outdoor Exposure Test Method) is 0.21 mdd, and the annual average sulfur oxide amount is 0.058 mdd. Yes. Although there is no actual measured weather condition, referring to the data of Tateyama Meteorological Office, the annual average temperature during the exposure period is 16.7 ° C, the annual average humidity is 73.0%, the annual precipitation is 1420mm, and the average annual wind speed is 2. It is estimated to be about 9m / sec.

  For details on how to determine the amount of aluminum nitride in steel (hereinafter referred to as α), please refer to [The Japan Iron and Steel Institute, Joint Research Group, Steel Analysis Group, “Analytical Techniques in the Japanese Steel Industry” (1982)]. The extraction residue analysis method with the following description is used. These nitrides in steel are, for example, 10% acetylacetone-1% tetramethylammonium chloride-methanol solution (hereinafter referred to as 10% AA-based solution), 4% sulfosalicylic acid-1% lithium chloride-methanol solution (hereinafter, 4%). A potentiostatic electrolysis method using an organic solvent such as a 6% bromine-methyl acetate solution or a 14% iodine-methanol solution, for example, a halogen organic solvent method in which the steel material is dissolved in a 6% bromine-methyl acetate solution or a 14% iodine-methanol solution. Can be separated as an extraction residue. For analysis of aluminum nitride dissolved in an alkaline aqueous solution in a short time, it is preferable to take a method in which an extraction residue is dispersed in an aqueous sodium hydroxide solution and nitrogen in the effluent is analyzed to convert it into an aluminum nitride amount.

In the specified range of Si in the steel material having the components shown in the present invention, the amount of Si ₃ N _{4 in} steel that causes an error in the alkali melting method is negligible compared to the amount of AlN in steel. However, if the residue is dispersed in a very high concentration sodium hydroxide aqueous solution for a long period of time, Si ₃ N _{4 with} a low dissolution rate will be eluted, and an analysis error will easily occur. is necessary.

  In the present invention, by using the amount of AlN converted based on the nitrogen analysis value of the effluent obtained in the step mainly comprising the electrolytic extraction method with 10% AA-based solution and the alkali dissolution method of the residue, A regulation relating to the amount of aluminum nitride (α / mass ppm) in steel was prepared. This chemical analysis process is complicated and sophisticated, and requires advanced knowledge and skills to operate.

  In addition, the quantitative determination of aluminum nitride in the extraction residue can be performed by a quantitative X-ray analysis method using ZnO or the like as an internal standard agent. After thoroughly pulverizing and mixing ZnO to a certain ratio (10% as an example) in the nonmagnetic extraction residue using an agate mortar, ZnO appearing at a diffraction angle 2θ = 36.44 ° obtained by X-ray diffraction By determining the peak height of AlN appearing at 2θ = 33.4 ° with reference to the peak height, the aluminum nitride can be quantified.

  Since it takes time to dissolve a large amount of steel required for analysis in the above-mentioned solvent, in order to enable analysis even with a small amount of nonmagnetic extraction residue, use a high-intensity X-ray source obtained by synchrotron radiation. You can also. In addition, there is a quantitative analysis method using a transmission electron microscope. Based on the electron diffraction pattern and particle shape information of various fine particles that make up the extraction residue, identification of more than 1000 arbitrarily selected non-magnetic extraction residue fine particles by diffraction pattern and particle shape observation was performed. The volume sum is obtained from the particle major axis length according to each classification of the identified substance, converted to mass by specific gravity, the mass ratio of aluminum nitride in the extraction residue is obtained, the mass of the steel before extraction and the total of the obtained extraction residue Convert to α (mass ppm) using mass data.

  In performing these physical analysis operations, it is preferable to obtain a calibration curve by correlating with the amount of aluminum nitride obtained by the above-mentioned standard method using a powder artificially mixed in advance. Analyzes related to other steel components can be easily obtained by anyone with a highly reliable analysis result by requesting a specialized analysis company.

  In order to make the effects of the present invention easier to understand, an example of the experimental results with the 3% Ni-0.35% Cu added weathering steel in Table 1 is shown in FIG. The comparative example which becomes a base component system is steel material number 30N-01 in Table 1, and the invention example is steel material number 30N-04 in Table 1. Since the elapsed years X (years) and the amount of corrosion Y (mm) are log-log plotted, the A value and B value shown in Equation 2 are the intercept on the Y axis of X = 1 year and the slope of the regression curve, respectively. Can be grasped as. Thus, the rust stabilizing ability as used in the present invention is the slope of the regression curve, that is, the B value, and it can be seen that if the B value is reduced, the corrosion amount after 100 years is greatly different.

  In Tables 1 to 8, the regression predicted corrosion amount ratio after 100 years is indexed as a β value based on the comparative base component steel. If β is less than 1 as a guide, it can be determined that the effect of the present invention was achieved. When a predetermined amount of aluminum nitride is contained and the oxygen content satisfies a predetermined value or less, the β value is generally less than 1. In the table, the addition of AlN molten steel means that a predetermined amount of AlN powder wrapped in aluminum foil in advance is placed in the mold in consideration of the yield, and the molten steel produced by a vacuum induction heating melting furnace is injected into the mold. It means steel.

  In this example, a predetermined amount of C, Si, Mn, etc. was added in the melting stage at a vacuum degree of 200 torr to deoxygenate, and the formed oxide was floated from the molten steel, and kept at 1600 ° C. to a vacuum degree of 200 torr or less. After leaving for 20 minutes or more, in order to increase the nitrogen yield, the pressure was restored with 99.9% pure nitrogen gas to 500 torr, the mold was poured, cooled and solidified to obtain a slab. In addition, although the capacity | capacitance of the melting furnace used by the present Example is 50 kg, it can manufacture even if it is the capacity | capacitance other than that.

  In actual steelmaking, aluminum nitride can be added to molten steel by the various means described above. The aluminum nitride powder to which molten steel is added has a high α value and a low β value, and the rust stability of the weather resistant steel material is higher. When the average particle long axis length of the aluminum nitride powder added to the molten steel was changed variously, differences were found in the rust stabilization ability, and the results are shown in Tables 9 to 10. The measurement of the particle major axis length and volume fraction of aluminum nitride in steel was performed using the transmission electron microscope described above.

  From this result, the particle major axis length of aluminum nitride in steel should be in the range of 1 μm or less and 0.001 μm or more, desirably 0.5 μm or less and 0.005 μm or more, and more desirably 0.3 μm or less. When it is in the range of 005 μm or more, it can be seen that there is an effect in improving rust stabilization ability.

It is drawing explaining the example of an effect of this invention.

Claims (6)

% By mass
C: 0.03% to 0.18%,
Si: 0.1% to 0.65%,
Mn: 0.2% to 1.4%
P: 0.03% or less,
S: 0.02% or less,
Cu: 0.3% to 2%,
Ni: 0.2% to 6%,
N: 0.002% to 0.01%,
Al: 0.01% to 0.5%,
Rust stabilization characterized by containing 5 to 50 ppm by mass of aluminum nitride containing 0.005% or less of O 2, the balance Fe and inevitable impurities, and a particle major axis length of 0.001 to 1 μm Weatherproof steel with enhanced performance. In mass%,
P: more than 0.03% to 0.2%,
Cr: 0.1% to 0.75%
Mo: 0.1% to 0.5%,
Ti: 0.001% to 0.03%
The weathering steel with enhanced rust stabilization ability according to claim 1, comprising at least one of the above. In mass%,
V: 0.001% to 0.05%,
Nb: 0.001% to 0.05%,
W: 0.001% to 0.05%
The weatherproof steel with enhanced rust stabilization ability according to claim 1 or 2, wherein any one or more of them are contained. In mass%,
Ca: 0.0001% to 0.005%,
Mg: 0.0001% to 0.005%
1 or 2 types of these, Weatherproof steel which improved the rust stabilization ability of any one of Claims 1-3 characterized by the above-mentioned.   The method for producing weatherable steel with improved rust stabilization ability according to any one of claims 1 to 4, wherein aluminum nitride is added to molten steel.   The rust stabilization according to any one of claims 1 to 4, wherein aluminum and nitrogen are added to the molten steel after deoxidation, or aluminum, nitrogen and aluminum nitride are added. A method for producing weathering steel with increased chemical capacity.

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